In the process of shaping steel it is usually squeezed through rolls to reduce its thickness. The rolls and associated structure are known in the parlance of the steelmaker as rolling mills or mills. Rolling mills may be classified variously and including (1) single direction, multiple stand mills wherein the successive reductions take place at each stand as the steel progresses through the mill; and (2) reversing mills which often comprise a single stand wherein the steel is passed back and forth through the single stand until successive reductions take place on each pass.
This invention relates to a reversing mill which comprises at least one stand between two driven coilers. The steel must pass back and forth in order to obtain the required reduction. After, perhaps, one pass the steel is thin enough to coil and may then be referred to as steel strip.
Reversing mills are inherently slow and the ends of the steel strip being passed through the mill tend to lose more heat than the remainder of the strip. (The ends are referred to as the head and tail depending upon the direction the steel strip is being processed through the mill.) Due to this cooling, the steel becomes harder near the ends, thereby increasing the resistance to deformation during rolling and the thickness or gauge of the strip increases near the ends (assuming the pressure applied remains constant from end to end of the strip). One established practice has been to automatically taper the ends of the steel in at least one of the early passes by adjusting the roll gap. Ideally, tapering begins at about the location where cooling begins to effect the hardness of the steel and continues until the end. While the tapering could be effected by increasing the roll gap as the head passes therethrough, it is normally effected by decreasing the roll gap as the tail passes therethrough. According to the prior art practice, the start of the taper is arbitrarily begun at a certain distance along the strip before the end thereof. This distance is measured by automatically counting the number of wraps that are collected on the take-up coiler. Simple calculations (given the nominal strip thickness and the reel diameter) enable the establishment of the distance remaining to the end of the strip. Also, according to the prior art, the taper is linear from the start of tapering to the end of the strip. While this technique has its decided advantages and has been used for many years with reversing mills, it also has an unfortunate disadvantage. The start of taper may come sooner or later than required to taper that portion of the strip effected by the end cooling. Also, it assumes that the cooling effect is uniform or a linear function of the distance to the end of the strip. In fact, cooling is not a linear function of the distance to the end of the strip.
Referring now to FIGS. 1, 2 and 3, the relative gauge error as determined by X-ray, the temperature of entry into the mill and the total mill load (the roll separating force to be explained) for one pass of an untapered strip are plotted. The figures rather dramatically illustrate the effect of cooling on gauge thickness. They also show that cooling (and therefore thickness) is not a linear function of distance to the end of the strip. Of course, temperature and strip thickness will vary from strip to strip. As the composition of the steel changes, so does the hardness at given temperatures. Nevertheless, FIGS. 1, 2, and 3 are typical and illustrative.